The refining of petroleum crude oils to obtain lubricant-stocks is based primarily on a series of steps including distillation, solvent refining and dewaxing.
For the preparation of a high grade distillate lubricating oil stock, the current practice is to vacuum distill an atmospheric tower residuum from an appropriate crude oil as the first step. This step provides one or more raw stocks within the boiling range of about 450.degree. to 1050.degree. F. After preparation of a raw stock of suitable boiling range, it is extracted with a solvent, e.g., furfural, phenol, n-methyl pyrrolidone, sulfolane, or chlorex, which is selective for aromatic hydrocarbons, and which removes undesirable components. The raffinate from solvent refining is then dewaxed, for example, by admixing with a solvent such as a blend of methyl ethyl ketone and toluene. The mixture is chilled to induce crystallization of the paraffin waxes which are then separated from the raffinate. Sufficient quantities of wax are removed to provide the desired pour point for the raffinate.
Other processes such as hydrofinishing or clay percolation may be used if needed to reduce the nitrogen and sulfur content or improve the color of the lubricating oil stock.
In recent years catalytic techniques have become available for dewaxing petroleum stocks. Although some attention has been directed to treating gas oils and manufacturing specialty oils, primary interest has been and is the catalytic dewaxing and subsequent treatment of lube oil stocks. Processes relating to the dewaxing of gas oils and specialty oils are described in U.S. Pat. Nos. 3,894,938 and 4,137,148.
U.S. Pat. No. 3,894,938 discloses a catalytic dewaxing process in which high-pour-point, high-sulfur gas oils having a boiling range of about 400.degree. F. to 900.degree. F. are first contacted, in the presence or absence of added hydrogen, with a ZSM-5 type zeolite hydrodewaxing catalyst which may contain a hydrogenation/dehydrogenation component. The effluent therefrom is subsequently desulfurized and denitrogenated by contacting it with a cobalt-molybdenum-alumina catalyst.
U.S. Pat. No. 4,137,148 discloses a process wherein specialty oils of low pour point and excellent stability are produced from waxy crude distillates having a boiling range of 450.degree. F. to 1050.degree. F. by solvent refining, catalytic dewaxing over a zeolite catalyst such as ZSM-5, and hydrotreating. The catalytic dewaxing reaction produces olefins which would impair properties of the dewaxed oil product if retained. These are saturated by hydrogenation in the hydrotreater, as confirmed by chemical analysis of the hydrotreated product for bromine number. Low bromine numbers are an indication of a satisfactory level of saturation. The hydrotreating step constitutes cascading effluent from the catalytic dewaxing step into a hydrotreating reactor of the type now generally employed for the finishing of lubricating oil stocks. Any of the known hydrotreating catalysts consisting of a hydrogenation component on a non-acidic support can be employed, for example, cobalt-molybdate or nickel-molybdate or molybdenum oxide, on an alumina support. Subsequent to this treatment, the effluent of the hydrotreater is topped by distillation to meet flash and firepoint specifications.
Techniques for dewaxing and subsequent treating of lubricating oil stocks are exemplified in U.S. Pat. Nos. 3,755,138 and 4,222,855.
U.S. Pat. No. 3,755,138 discloses a process wherein a lube oil stock boiling between 650.degree. F. and 1100.degree. F. is subjected to mild solvent dewaxing and subsequently to hydrodewaxing. The hydrodewaxing step constitutes contacting the lube oil stock with a crystalline aluminosilicate of the ZSM-5 type which contains a metal hydrogenating component in the presence of added hydrogen. In U.S. Pat. No. 4,222,855 lube oil stocks boiling between 600.degree. F. and 1050.degree. F. are catalytically dewaxed by contacting them with a crystalline aluminosilicate having particularly characterized pore openings such as ZSM-23 and ZSM-35.
U.S. Pat. No. 3,668,113 discloses a process in which petroleum fractions such as gas oil and wax distillate fractions are first passed over a catalyst comprising a crystalline mordenite of reduced alkali metal content and a metal hydrogenating component to remove wax. The reaction product is then passed over a catalyst comprising a refractory inorganic oxide support and a hydrogenating component selected from metals and compounds thereof of Groups VI and VIII of the Periodic Table to remove sulfur.
A currently preferred process is one wherein the actual dewaxing is accomplished in a first reactor using a zeolite catalyst such as the ZSM-5 zeolite and then hydrofinishing the dewaxed effluent in a second reactor in order to reduce olefinic compounds produced in the first reactor. The hydrofinishing step produces a base stock having an acceptable oxidative stability.
A problem associated with the dewaxing step in the first reactor is that channeling or bypassing might occur in which a small portion of the waxy raffinate from the solvent refining step flows almost directly through the reactor bed with minimal contact with the catalyst. Consequently, little if any, dewaxing occurs in this portion of the raffinate. The resulting product may not meet critical performance tests even though pour point or cloud point specifications are met. Another possible problem can occur in the heat exchange system. Ordinarily the system is designed to transfer heat from the heated dewaxed product stream downstream to the cooler incoming waxy raffinate upstream where the pressure is higher. If any leaks across the physical barriers separating the two systems occur, the product effluent (the dewaxed product) will be contaminated and be of unacceptable specifications.